This month, the 7 billionth person will be born on a planet already strapped for resources. To mark this extraordinary milestone, TIME explores the most pressing population issues of the day, from the quest for sustainable energy sources to a look at what our biggest cities could look like in the decades to come

8. Delhi

By Vanessa KoWednesday, Oct. 26, 2011

Manish Swarup / AP

Current population: 18.6 million

Between the downtown core and its vast network of city-size suburbs, the cosmopolitan Indian capital is home to some 18.6 million. Like many fast-growing cities, Delhi has struggled to accommodate waves of migrants from rural areas, leaving millions to live in unplanned shantytowns where water, electricity and proper sewer systems are spotty. The city is famous for its traffic jams, but public transportation isn't always an option: the train system is supercrowded. Deadly stampedes at train stations have led to calls for urgently needed infrastructure improvements.